Power converters, such as AC to DC converters or DC to AC inverters, generally comprise networks of parallel and/or series connected power switching devices such as insulated gate bipolar transistors (IGBTs). Such converters may be for applications ranging from low voltage chips, to computers, locomotives and high voltage transmission lines. Converters may be used for example for switching in high voltage dc transmission lines of the type which may, for example, carry power from an offshore wind installation, and for medium voltage (for example greater than 1 kV) switching for motors and the like, for ex-ample locomotive motors.
The following description generally relates to driving one or more power switch(es), which in turn are configured for driving a load of a power converter. As the power switch(es), IGBTs are used for example to control large currents by the application of low level voltages or currents, some IGBTs having ratings of, e.g., 1200 V or 1700 V, and/or 1200 A. However, principles and embodiments described herein are generally applicable where the power switch(es) are instead a MOSFET such as a silicon carbide MOSFET (vertical or lateral), HEMT, JFET or other type. Thus, any mention of IGBT or power switch can generally be replaced with reference to any such device. Furthermore, the techniques we will describe are not limited to any particular type of device architecture and thus the power switching devices may be, for example, either vertical or lateral devices; they may be fabricated in a range of technologies including, but not limited to, silicon, silicon carbide or gallium nitride.
Nevertheless, the power semiconductor switching devices with which we are concerned typically have a current carrying capability of greater than 1 A and are operable with a voltage of greater than 100 V, for example devices that are able to carry currents of greater than 10 A, 50 A or 100 A and/or are able to sustain a voltage difference across the device of greater than 500 V or 1 kV.
A typical IGBT gate drive comprises the elements shown in FIG. 1. The gate drive logic comprises digital logic circuits reference to a 3V3 or 5 V power supply which receives an incoming signal (PWM) indicating when to switch the power switch (e.g., IGBT) ON and OFF. The gate drive logic creates signals (SOURCE and SINK) indicating when current is to be supplied to and removed from the power switch respectively. A level translation stage is typically required to drive the power switch over a wider voltage range, e.g., −10 V to +15 V. The output stage for driving an IGBT comprises transistors P and N channel MOSFETS (labelled PMOS and NMOS) or bipolar PNP and NPN transistors with a turn-ON resistor (Ron) and a turn-OFF resistor (Roff) which are chosen to match the characteristics of the power switch and/or load. The output stage transistors can handle high current and normally require a drive stage as the digital logic and level translation cannot provide enough current to turn them on and off directly.
An IGBT may be provided in a module preferably including the IGBT device and a commutation diode, i.e., free-wheeling diode, in antiparallel. IGBT module manufacturers generally publish preferred gate resistance values for minimal losses. In the case of IGBT turn-on there is a trade-off between IGBT switching loss and diode reverse recovery loss. It is generally desired that any attempts to reduce the overall losses ensure that the diode stays within its safe operating area (SOA), which may be represented on a graph of voltage against current by a line of maximum power dissipation which is preferably not crossed. The diode may be damaged if the switching speed of the converter output circuit is too fast, for example. IGBT manufacturers generally optimise their products for minimal overall losses assuming a voltage source drive with resistance (resistive drive). For this reason it is desirable that any drive circuit (driver) is a resistive drive preferably with output (gate drive) resistance within an IGBT manufacturer's specified range.
To reduce conduction losses it is desirable for a power switch control terminal, e.g., the IGBT gate, to be held at the highest possible voltage when the device is on. Device datasheets usually state 15 V as the normal operating point with absolute maximum at 20 V. Silicon carbide (SiC) MOSFETs on the other hand are usually expected to be operated at 18 to 20 V with a higher absolute maximum. To make a universal gate drive it is desirable to be able to configure the gate voltage.
Most power semiconductors have a short-circuit withstand capability. This is a time (typically 10 s) during which the device can withstand excessive current without failure. It is desirable that the gate drive can detect this condition and turn-off the IGBT safely within this time. The time is usually specified at a particular gate voltage.
In order to achieve the 10 μs short-circuit rating, IGBT manufacturers often trade-off conduction losses or silicon area. In other words higher performance devices could be realised if the 10 s requirement was relaxed. Improved measurement and control circuits are desirable to give device manufacturers the opportunity to create higher performance devices that can still be protected under abnormal conditions.
An improved method of controlling voltage levels on a power switch control terminal is therefore desired, for example to provide advantages such as, inter alia, reduced cost, reliability, low circuit complexity, low component count, and/or lower power dissipation, etc.
For use in understanding the present invention, the following disclosures are referred to:
DE 10 2006 034 351 A1; and                “Advantages of Advanced Active Clamping”, Power Electronics Europe, Issue 8 2009, pp. 27 to 29.        